TV Graphics and mixing control

ABSTRACT

A system employed in a television receiver for decoding and displaying graphics information such as may be encoded on a broadcast video signal. The system includes a graphics signal decoder supplied with the encoded video signal, and a plurality of transmission gates responsive to logic control signals derived from outputs of the decoder. The gates are coupled to inputs of video output stages in the receiver, and operate in response to the control signals for interrupting normal video signals to the video output stages and for enabling a voltage representative of a graphics display intensity level to be coupled to the video output stages when graphics information is to be displayed.The system also includes a graphics level control network. The level control network limits excessive graphics representative beam current levels to prevent image defocusing and kinescope screen burn. The level control network also adjusts the graphics display intensity level in accordance with the level of image representative video signals in a mixed video plus graphics display mode, to preserve a desired contrast relationship between the displayed video and graphics information.

This invention concerns an arrangement for displaying graphics oralphanumeric information by an image reproducing kinescope in atelevision receiver or equivalent video signal processing system. Inparticular, the invention concerns such an arrangement to facilitatecoupling of graphics signals to video signal processing stages of thereceiver, and to automatically adjust and limit the intensity ofdisplayed graphics information.

A color television receiver, for example, can be arranged to displayeither normal video information alone in a conventional manner, graphicsinformation alone (e.g., "video games" or alphanumeric data displays),or mixed video and graphics information (e.g., superimposed subtitles,weather, sports or road traffic information). Graphics informationsignals can be advantageously provided in a "Teletext" system forexample, which involves transmitting alphanumeric and other graphicsinformation through conventional television transmitting equipment, andreceiving, decoding and displaying the graphics information by means ofa conventional television receiver. A Teletext graphics signal comprisescoded digital information which is sent as a series of digital addresscodes, during two horizontal line periods, towards the end of thevertical blanking period of the composite video signal. In accordancewith one such system, lines 17 and 18 in one field and lines 330 and 331in another field are used. Additional details of such a Teletext systemare contained in an article entitled "Teletext Data Decoding - The LSIApproach" by B. Norris and B. Parsons, published in IEEE Transactions onConsumer Electronics, pages 247-252, August 1976, and in a "BroadcastTeletext Specification" (September 1976) published by the BritishBroadcasting Corporation.

In one graphics display system employing the "TIFAX XM11" decoderavailable from Texas Instruments, Ltd. of Bedford, England, as describedin application report No. B183 for this decoder, graphics informationsignals are supplied directly in amplified form as high level driveinputs to video output stages of the receiver. In contrast to thisapproach, in accordance with the principles of the present invention, aplurality of digitally controlled transmission gates is employed forswitching a bias voltage representative of a graphics display intensitylevel to the video output stages when graphics information is to bedisplayed. This approach advantageously permits the graphics informationto be supplied to standard video signal processing stages such as videooutput stages so that the number of required interfacing elements andcircuit modifications are minimized, signal loss is minimized, and highfrequency response is preserved to maintain good image definition.

In a graphics display system, excessive kinescope beam current levelsproduced in response to kinescope drive signals can cause imagedefocusing which can distort or obscure small text or symbols, or whichcan cause "screen burn" when stationary patterns are displayed. Also, inthe mixed display mode when graphics information is displayed togetherwith normal television picture information, the degree of contrastbetween the graphics and picture information can vary due to variationsin the level of the television signal. Thus, displayed graphicsinformation may appear too intense (e.g., when the displayed televisionpicture is dark in the vicinity of the graphics information), orobscured (e.g., when the television picture is bright in the region ofdisplayed graphics information). It is herein recognized as desirable tolimit the intensity of displayed graphics information by limiting thesignal drive to the kinescope, and to automatically maintain apredetermined relationship between the intensity of displayed graphicsinformation and the variable level of the television signal.

Graphics signal display apparatus which facilitates interfacing thegraphics signal source with standard video signal processing circuits,and which includes provision for automatically controlling the level ofintensity of the graphics display, is provided in accordance with theprinciples of the present invention in a video signal processing systemincluding a video signal processing channel, an image display kinescope,and a network for coupling processed video signals from the videochannel to intensity control electrodes of the kinescope. Signalsrepresentative of graphics information to be displayed are derived froma source of signals containing graphics information, and a local biassource provides a graphics intensity bias signal representative of adesired level of displayed graphics information. A first switchingnetwork has a signal input terminal coupled to the video channel, asignal output terminal coupled to the coupling network, and a switchingcontrol input terminal. A second switching network has a signal inputterminal for receiving the graphics bias signal, a signal outputterminal coupled to the coupling network, and a switching control inputterminal. Switching control of the first and second switching networksis provided by output signals from a control circuit which responds tothe derived signals. In a normal operating mode when video signals aloneare to be displayed, the first switching network conducts video signalsto the coupling network, and the second switching network inhibitsconduction of the graphics bias signal to the coupling network. In agraphics display operating mode, the first switching network inhibitsconduction of video signals to the coupling network, and the secondswitching network enables the graphics bias signal to be conducted tothe coupling network during graphics display intervals.

In accordance with a feature of the invention, the bias source iscoupled to the video channel for monitoring the level of the videosignals, and includes means for varying the magnitude of the graphicsintensity bias signal in accordance with the level of the video signalsin a mixed graphics display mode, to thereby maintain a desiredrelationship between the intensity of displayed graphics information andthe level of the video signal.

In accordance with a further feature of the invention, the bias sourceincludes means for establishing minimum and maximum levels of thegraphics intensity bias signal, and for controlling the level of thegraphics bias signal in a direction to reduce kinescope currentconduction under conditions which would otherwise cause excessivekinescope current conduction.

In the drawing:

FIG. 1 shows a portion of a color television receiver partly in blockdiagram form and partly in schematic circuit diagram form, including agraphics display arrangement according to the present invention;

FIG. 2 shows additional details and features of the arrangement of FIG.1;

FIG. 3 depicts an additional circuit feature of the arrangement shown inFIG. 2;

FIG. 4 illustrates a displayed graphics symbol illustrative of a featureof the present invention; and

FIG. 5 shows waveforms illustrative of the timing relationships ofsignals utilized in the arrangement of FIG. 2.

In FIG. 1, a source of luminance signals 10 in a luminance channel of acolor television receiver provides an output luminance signal Y, and asource of color difference signals 12 in a chrominance channel of thereceiver provides output color difference signals R-Y and B-Y. Sources10 and 12 include conventional television signal detecting, amplifyingand other signal processing circuits. The luminance and color differencesignals are combined in a matrix 13 for providing R, B and G color imagerepresentative output signals. These signals are applied to respectiveintensity control electrodes of a color kinescope 15 via a video outputstage 16, and a switching stage 20. Video output stage 16 may comprise aplurality of cascode video amplifiers of the type described in U.S. Pat.No. 4,051,512, for example.

The graphics display system includes switching stage 20, a switchinglogic control unit 22, and a level control circuit 25, and a graphicssignal source 27 which provides decoded graphics information signals andcontrol signals. In this example, graphics signal source 27 correspondsto the "TIFAX XM11 Teletext Decoder" available from Texas Instruments,Ltd. of Bedford, England.

Graphics source 27 receives Teletext encoded video signals at an inputterminal 16 from a source of video signals 28 (e.g., from the output ofa video detector stage in the receiver), and also receives horizontalline flyback synchronizing pulses such as may be derived from horizontaldeflection circuits of the receiver at an input terminal 15. Source 27provides digital output signals for ultimately determining the signalsapplied to kinescope 15 during graphics display intervals. An outputBLANKING control signal from a terminal 21 of source 27 is applied tologic unit 22, level control circuit 25, and also to video blankingcircuits of the receiver. Analog video signals are absent or blankedwhen the receiver is used to display graphics alone in the "graphicsonly" mode. Digital R, G and B signals from output terminals 19, 18 and17 of graphics source 27 are representative of the graphics informationto be displayed, and are provided from source 27 either singly or incombination. A MONO digital control signal from an output terminal 20 ofunit 27 is utilized to disable the transmission of the analog R, G, Bcolor signals from matrix 13 to kinescope 15 in order to make displayspace available for graphics information in the mixed display mode. Thissignal exhibits a duration which corresponds to an interval during whichthe graphics information is present in the mixed display mode. Theterminal numbers shown within block 27 correspond to the actual numbersof the external signal terminals of the TIFAX XM11 decoder. Moredetailed information concerning the TIFAX XM11 decoder is contained inApplication Report No. B183 for this decoder.

The digital control signals from graphics source 27 are processed bylogic unit 22, which in turn develops output digital control signals forcontrolling the operation of switching unit 20. Switching unit 20comprises an array of analog signal transmission gates (electronicswitches) 31-36 for switching the signal inputs of the video outputamplifiers within output stage 16 between normal analog televisionsignals and graphics signals. Each gate is a three-terminal devicehaving a signal input terminal, a signal output terminal, and a controlterminal to which digital signals are applied for controlling the on/offoperation of the gate.

In the normal operating mode of the receiver when it is desired todisplay only analog television signals, gates 31, 32 and 33 are "closed"(i.e., rendered conductive) to permit the analog R, G, B signals frommatrix 13 to be conducted to kinescope driver 16. This is accomplishedin response to an enabling digital control signal from a terminal 41 oflogit unit 22, which is coupled in common to the control terminals ofgates 31-33. At the same time, gates 34, 35 and 36 are "opened" (i.e.,rendered nonconductive between the signal input and output terminals) inresponse to disabling digital control signals respectively applied tothe control terminals of these gates from output terminals 42, 43 and 44of logic unit 22. When graphics information is to be displayed, digitalsignals, from graphics source 27 and logic unit 22 cause gates 31-33 toopen, thereby preventing analog television signals then present frombeing conducted to kinescope 15 during the graphics interval. Logic unit22 provides one or more digital control pulses from terminals 42-44during this interval, causing one or more of gates 34-36 to close. Thoseof gates 34-36 which close couple the associated R, G or B input ofkinescope driver 16, and thereby the associated intensity controlelectrode of kinescope 15, to a graphics intensity bias voltage which issupplied as a common input to gates 34-36 from level control unit 25, aswill be discussed. The kinescope screen is then illuminated in responseto this bias voltage during the graphics interval.

Switching stage 20 and logic unit 22 are shown in greater detail in FIG.2, which will be discussed subsequently.

Continuing with FIG. 1, level control circuit 25 serves to automaticallycontrol the intensity of the displayed graphics information inaccordance with the level of the R, G, B signals from matrix 13 duringthe mixed display mode, since the legibility of displayed graphicscharacters is dependent on television image content in the mixed mode.For example, white graphics characters would be difficult to discernagainst a light image background.

The R, G, B signals from matrix 13 are combined by means of positivepeak rectifier diodes 47, 48 and 49 to form a combined signal with amagnitude corresponding to the peak level of the R, G, B signals. Thecombined signal is coupled to an input of level control circuit 25 andserves to vary the base bias of a transistor 50 in accordance with themagnitude of the combined signal. Transistor 50 develops a correspondingemitter current (I_(C)) which charges a storage capacitor 52. A timeconstant determined by the value of capacitor 52 and a bleeder resistor55 is slightly greater than the time of one image scanning field so thata voltage developed on capacitor 52 closely approaches and tracks withthe level of the combined analog signal. A bleeder resistor 55 providesa path to ground for capacitor discharge current I_(D). A resistor 53serves as a current limiting resistor for collector current oftransistor 50.

The voltage developed on capacitor 52 is translated via a D.C. coupledPNP follower transistor 57, series diodes 58, 59 and an NPN followertransistor 60, and appears across a resistor 63 in the emitter circuitof transistor 60 as a graphics intensity bias control voltage V_(A).Voltage V_(A) is coupled via a current limiting resistor 64 to thesignal inputs of gates 34-36 in switching stage 20, for determining theintensity of displayed graphics information. Since voltage V_(A) varieswith the level of the combined R, G, B signals, the intensity ofdisplayed graphics information also varies with the R, G, B signals. Itis noted that the offset voltage drop associated with diodes 47, 48, 49and the base-emitter junction offset voltage of transistors 50, 60 arecompensated for by the combined effect of the base-emitter junctionoffset voltage of transistor 57 and the offset voltage across diodes 58,59. Thus the peak-representative voltage level at the emitter output oftransistor 60 is substantially equal to the peak value of the combinedanalog signal from diodes 47, 48 and 49, and is also compensated fortemperature variations by means of the offset voltage drops noted above.

When the television signal is representative of a dark scene with a lowbrightness level in the mixed display mode, it is desirable for theintensity of the displayed graphics information to exceed the lowbrightness level of the television image. In this instance, the chargeon capacitor 52 is determined by a voltage divider comprising resistors66, 68 and an adjustable resistor 69 coupled to a source of directvoltage (+12 volts). Resistor 69 is pre-set to establish a minimum levelof graphics display intensity. When the television signal isrepresentative of scenes of average or high brightness, a diode 70 isreverse biased (non-conductive) and resistors 66, 68 and 69 aredecoupled from transistor 50 and capacitor 52. Capacitor 52 then chargesas discussed above. Under low brightness conditions, however, the basevoltage of transistor 50 becomes sufficiently less positive so thatdiode 70 becomes forward biased into conduction, thereby permittingcapacitor 52 to charge via resistor 66, diode 70 and transistor 50 to alevel greater than otherwise would have occurred under low brightnessconditions. Accordingly, voltage V_(A) and thereby the intensity of thedisplayed graphics information are maintained at a desired minimumacceptable level in the mixed display mode. The intensity of displayedgraphics information is less critical in the "graphics only" displaymode, when there is no displayed analog picture information surroundingthe graphics information. The graphics intensity level in the "graphicsonly" mode is set by potentiometer 76 as discussed below.

In the "graphics only" display mode, the BLANKING signal from source 27comprises a fixed positive D.C. level which causes the video blankingcircuits of the receiver to inhibit the analog television signals, andwhich causes transistors 71 and 72 of circuit 25 to conduct with theresult that a bias voltage is developed across potentiometer 76. In themixed display mode, however, the BLANKING signal comprises a series ofshort duration positive pulses which correspond to the conventionalhorizontal retrace blanking and vertical retrace blanking pulses. TheBLANKING signal remains at a "low" level during television image linedisplay intervals, at which time the receiver blanking circuits permitthe analog television signal to be processed normally and coupled tomatrix 13. The blanking pulses developed in the mixed display mode areprevented from activating transistor 71 in level control circuit 25 bymeans of a low pass RC filter including a filter capacitor 29 coupled toa base input of transistor 71. This filter serves to filter out themixed display mode blanking pulses, rendering transistors 71 and 72nonconductive in the mixed mode, and thereby preventing a bias frombeing developed across potentiometer 76 during the mixed mode. Thegraphics display operating bias in the mixed display mode is obtained asexplained above.

In the "graphics only" display mode, the D.C. level blanking signal fromsource 27 causes transistor 71 to conduct. A PNP transistor 72 alsoconducts in response to the conduction of transistor 71, and develops acollector voltage for biasing a voltage divider network comprising anadjustable resistor 75, potentiometer 76 and a resistor 77 arranged inseries in the collector circuit of transistor 72. A diode 80 becomesforward biased in response to a voltage then developed at a wiper ofpotentiometer 76, and voltage V_(A) at the output of circuit 25substantially equals the voltage appearing at the wiper of potentiometer76. Resistor 75 is pre-set to limit the intensity of the displayedgraphics information and thereby the magnitude of kinescope beam currentto a level which is not expected to damage the kinescope display screen(i.e., due to "burn-in" effects). Potentiometer 76 is an optionalcontrol to permit a viewer to adjust the maximum graphics intensity tosome other level if desired.

Kinescope beam current associated with the displayed graphicsinformation must be limited at a safe long-term average level, and alsoat a substantially higher short-term peak value. The peak value chosentypically represents a compromise between a current level associatedwith a desired peak level of graphics display intensity, and a levelwhich does not produce objectionable spot blooming with attendant lossof image definition. Level control circuit 25 is arranged so that theopen circuit value of voltage V_(A) corresponds to a desired peakgraphics drive level to the input of video output stage 16. Shortduration graphics representative current peaks into video stage 16 aresupplied from a charge storage capacitor 65, which is regularly beingrecharged from circuit 25 through a resistor 64. The average graphicsrepresentative current is normally significantly less than the peakcurrent and, therefore, the normal charge on capacitor 65 closelyapproximates the open circuit level. However, if a large amount ofgraphics information is displayed, the average beam current will behigh. The graphics representative current then supplied to video outputstage 16 will also be large, and a significant voltage drop is developedacross resistor 64. The value of resistor 64 is chosen so that thisvoltage drop reduces the effective value of voltage V_(A) to a levelwhich produces a safe level of average kinescope beam current.

In practice, a color television receiver typically includes provisionfor adjusting the video output stage for a desired threshold conductionlevel (black level), and for adjusting the signal gain (white balance)of the output stage in a service or set-up operating mode of thereceiver. This is typically accomplished by means of variable resistorsor potentiometers (not shown) associated with the video output stage.The graphics control system may also include provision for setting thenormal "white level" or intensity of displayed graphics information inthe graphics mode to correspond to the normal "white level" of the videosignal as determined by adjustment of the video output stage during theservice mode. This adjustment can be accomplished by means of optionaladjustable resistors (not shown) respectively connected between thesignal inputs of gates 34-36 and output voltage V_(A) of level controlcircuit 25.

It is noted that the described graphics display system advantageouslyutilizes the signal outputs of the graphics signal source (e.g., theXM-11 or other decoder) as transmission gate control or enabling signalsonly. This is in contrast to graphics display systems which employ thegraphics information signals in amplified form as a direct high levelvideo drive to the video output stages, which results in addedcapacitance at the video output stage and an attendant loss in highfrequency response. The described system also avoids the use of longvideo signal and return connections to the output stages, and thereforeavoids problems due to spurious radiations of video frequencies insensitive areas of the receiver. Interference due to signal frequenciesgenerated with the receiver (e.g., due to deflection signals and powersupply operation) is also minimized.

The input of the video output stage is an attractive interface point,since circuit impedances here are relatively low and stray capacitanceis less of a problem. The interface transmission gates simply enable ordisable the analog television signal and, in the latter case, the inputto the video output stage is switched between two bias levels. Highfrequency response and timing accuracy are preserved, and undesiredsignal cross coupling effects are minimized. All of this can beaccomplished in close proximity to the video output stage to minimizethe likelihood of spurious signal interference. If the outputs fromdecoder signal source 27 were used directly to provide low level videodrive signals, it may be difficult to maintain a precise white or blacklevel balance of the graphics information because of tolerance effectsor drift in the R, G, B decoder outputs. These difficulties are avoidedin the present system, since the output signals from signal source 27are used as logic control signals rather than as direct drive videosignals.

Since in the described system the decoder output signals are notamplified to an appropriate level for driving either the kinescopedirectly or preceding video processing circuits, the graphics videodrive level can be determined from within the graphics display system orfrom an appropriate source of bias potential within the receiver. Thisfacilitates the use of various types of decoders, since the level of thedecoder output signal is not critical.

Referring now to FIG. 2, there is shown additional details of switchingstage 20 and logic unit 22 of FIG. 1. Corresponding elements in FIGS. 1and 2 are identified by the same reference number.

Logic unit 22 includes a logic AND gate 210 responsive to the MONOsignal, and to an inverted BLANKING signal supplied from an output of aninverter 212. The MONO signal is present whenever R, G or B graphicssignals are present, and is utilized to open gates 31-33 for theduration that graphics information is to be displayed. The output fromAND gate 210 is coupled to the switching control inputs of gates 31-33for controlling the conduction status of gates 31-33. Gate 210 andinverter 212 are arranged so that a positive output level (logic "1" or+12 volts, for example) is provided from AND gate 210 only when the MONOsignal input to gate 210 is at a positive logic "1" level and theBLANKING signal input to inverter 212 is at a significantly lesspositive level (logic "0" or 0 volts, for example). A positive outputfrom AND gate 210 causes gates 31-33 to close so that the analog R, G, Bsignals are transmitted to video output stage 16 in normal fashion. Forall other conditions of the MONO and BLANKING signals, AND gate 210produces the less positive logic "0" output level which causes gates31-33 to open, thereby interrupting the transmission of signals R, G, Bduring the graphics interval. It is noted that the BLANKING signalremains at a "high" D.C. level in the "graphics only" display mode.Gates 31-33 remain permanently open in this mode, since an inverted("low") blanking level is applied to the control terminals of gates31-33 from the output of inverter 212 via gate 210.

Also during the graphics display intervals, one or more R, G, B digitaloutput signals (logic "0" levels) are supplied from graphics controlsource 27 to logic unit 22. These signals are inverted to a morepositive logic "1" level by inverters 214, 216 and 218, and as such arerespectively applied to the switching control terminals of gates 34-36for rendering one or more of these gates conductive to develop thegraphics display by permitting graphics bias voltage V_(A) to be coupledto one or more of the R, G, B inputs of video output stage 16, asmentioned in connection with FIG. 1.

When the graphics display interval ends, gates 34-36 return to the opencondition and, if the receiver is operating in the mixed display mode,gates 31-33 return to the closed position so that signal and bias levelsfor R, G, B video output stage 16 comes under control of the outputsignals from matrix 13. However, when the receiver is operating in the"graphics only" mode, all of gates 31-36 are open when the graphicsinterval ends. Gates 31-33 remain open in response to the BLANKING andMONO signals from source 27 to prevent the analog television signals ornoise from reaching video output stage 16.

The time between the end of the graphics interval (when the bias voltagesupplied via one or more of gates 34-36 is decoupled from the videooutput stage), and the time when the bias level appearing at the R, G, Binputs to video output stage 16 diminishes to a level corresponding toblack level, depends on a time constant formed by the input resistanceand stray capacitance associated with the input circuits of video outputstage 16 which were operative during the graphics display interval. Ifthis time constant and the associated "fall time" to black level are toolong, the trailing edge of the graphics display will appear to persistbeyond the graphics information interval (e.g., as a gray-scale smear).This effect distorts the desired contrast of the graphics display, andis eliminated by means of auxiliary gating stage 250 includingtransmission gates 252, 255 and 257 in cooperation with logic NOR gates224, 230 and 233 within logic unit 22. Gates 252, 255 and 257 areconnected between the R, G, B input lines to video output stage 16 and asource of low D.C. voltage V_(B) (i.e., a black reference level such asground or a source of reference bias potential for output stage 16). Theoutputs of NOR gates 224, 230 and 233 are respectively coupled to theswitching control terminals of transmission gates 252, 255, 257.

Gates 252, 255, 257 operate only in the "graphics only" mode and closewhen the display screen should be dark (i.e., immediately after thegraphics display interval ends) in response to the BLANKING signal fromsource 27, which in this mode is a D.C. level. When gates 252, 255, 257close (i.e., conduct), the input lines to output stage 16 are rapidlyconnected to voltage V_(B) so that any residual charge associated withthe input capacitance of the video output circuits is rapidly dischargedto the level of voltage V_(B), which preferably corresponds to a desiredblack level.

More specifically, the auxiliary gates are closed when the output of theassociated NOR gate is at a positive level, corresponding to a logic "1"level in this example. This condition is produced when both inputs to agiven NOR gate are at a logic "0" level at the end of the graphicsinformation display interval, when the inverted BLANKING signal at theoutput of inverter 212 is at a "0" logic level, and when the inverted R,G, B signals at the outputs of inverters 214, 216, 218 are also at a "0"level. The trailing edge of the graphics information pulses is fast dueto the low impedance path provided by the auxiliary gates between thevideo inputs of stage 16 and voltage V_(B). The required low impedancepath for black to white graphics information pulses is provided throughgates 34, 35, 36.

The timing of the switching of transmission gates 31-36 is important,since an image "dot" interval associated with small graphics characters(e.g., "Teletext" characters) can have a duration of approximately 180nanoseconds, and observations indicate that timing errors of thegraphics display control signals should be less than approximately fiftynanoseconds to avoid undesirable graphics edge effects. In addition, itis desirable to enhance the appearance of the displayed graphicsinformation when operating in the mixed display mode by surrounding eachdisplayed graphics character with a narrow black outline. In this regardit is noted that a displayed graphics character is contained within anarea sometimes referred to as a "blanking box", designating an areawhich is blanked or at black level except for the graphics informationdisplayed within this area. The image areas immediately preceding andfollowing each displayed graphic symbol are particularly significant forthe purpose of providing a desired amount of graphics contrast to permitthe graphics information to be more easily seen. A black outline of agraphics character can be produced as described below with reference toFIGS. 2-5.

With regard to FIG. 2, it is noted that the R, G and B switching controlsignals which enable (close) one or more of gates 34-36 during thegraphics interval are first passed through inverters 214, 216 and 218.These inverters introduce a small signal propagation delay (D₁ in FIG.5) of approximately twenty-five nanoseconds such that the time at whichgates 34-36 are closed is delayed by this amount relative to the time atwhich gates 31-33 open. Since video gates 31-33 open before graphicsgates 34-36 close, video information otherwise transmitted via gates31-33 is absent or blanked for a short interval before the graphicsdisplay information is transmitted via gates 34-36 to output stage 16.The leading edge of the displayed graphics information therefore appearsenhanced due to the small blanking or black level interval whichprecedes the leading edge of the graphics information.

The trailing edge of displayed graphics information can also be enhancedwith a black outline by slightly delaying the low (disable) to high(enable) switching transition of the digital control signal applied fromAND gate 210 to the switching control terminals of video gates 31-33. Byslightly delaying the time when gates 31-33 are enabled to conduct videosignals from matrix 13 after the graphics information has beendisplayed, the image area immediately following the trailing edge of thedisplayed graphics information will be at the blanking or black level,thereby resulting in a narrow dark outline of the trailing edge of thegraphics information. This result can be obtained by employing anauxiliary delay circuit 310 as shown in FIG. 3.

Delay circuit 310 includes a diode 312, a resistor 313 and a capacitance315 arranged as shown between the output of AND gate 210 and theswitching control input terminals of gates 31, 32 and 33. Network 310provides a propagation delay D as shown with respect to switchingcontrol or "masking" waveform A in FIG. 3 (see also FIG. 5), as follows.Capacitor 315 is normally charged to the operating supply (+) but israpidly discharged to a low potential through diode 312 and gate 210when gate 210 is activated. Gates 31-33 are then disabled(nonconductive). Capacitor 315 slowly recharges through resistor 313when gate 210 is inactivated. After a time delay D, the voltage oncapacitor 315 is sufficient to cause gates 31-33 to close.

FIG. 4 illustrates a displayed graphics symbol with edge enhancement asdiscussed above, and also shows the timing relationship between waveformA and the enhanced trailing edge of the displayed symbol. FIG. 5 isself-explanatory and illustrates the timing relationships between the R,G, B signals, the MONO signal, and the switching control output signalfrom AND gate 210, in the mixed display mode.

The circuit of FIG. 2 comprises standard CMOS logic components and canbe constructed from presently available integrated circuits.Illustratively, CMOS integrated circuit type CD4066 can be used toprovide transmission gates 31-36 and 252, 255, 257. Integrated circuittype CD4049 can be employed to provide inverters 212-218, while AND gate210 and NOR gates 224, 230, 233 can be provided by integrated circuittypes CD4081 and 4001, respectively. These integrated circuit types areavailable from the Solid State Division of RCA Corporation, Somerville,N.J. The circuit of FIG. 2 is capable of being fabricated in a singleintegrated circuit, as is level control circuit 25 with the possibleexception of capacitors 29, 52 and 65.

Although the invention has been described with reference to particularembodiments, various additional modifications can be made within thescope of the invention.

Additional analog input signals (e.g., from the R, G, B outputs of acolor camera or other local video information source) can also bedisplayed in the graphics mode, instead of digital graphics informationas discussed. In this alternative use, the respective signal inputs ofgates 34, 35 and 36 can be switched to receive voltage V_(A) in common(as discussed), or to receive the additional analog signals. In thelatter instance, voltage V_(A) would be decoupled from gates 34-36, andeach of these gates would be separately driven by separate ones of theadditional analog signals. Also, the R, G, B signal inputs of inverters214, 216, 218 and the MONO signal input of AND gate 210, and theBLANKING signal input of inverter 212 (FIG. 2) would be connected toground, for example, when the additional analog signals are utilized inthe graphics mode. An appropriate switching mechanism can be employedfor this purpose.

The described system is not limited to decoding and displaying"Teletext" encoded video signals. The system can also be used fordisplaying alphanumeric information derived from other signal sources,such as a personal "home computer", such as the RCA VIS personalcomputer system. The described graphics display system is attractive forthis purpose since the only external signals required are digital innature and the signal levels are not critical. The required graphicsbias signals for the video output stages are generated internally by thesystem, and image resolution is high since connections to narrow signalbandwidth portions of the receiver are not required.

The MONO signal can also be employed in conjunction with a monochromereceiver, as described in Application Report B183 for the TIFAX XM11Teletext Decoder, and can also be derived from the R, G, B signals fromsource 27 by means of suitable logic switching circuits. In addition,the level of intensity of displayed graphics information also can becontrolled in response to a conventional kinescope beam current controlsignal, such as can be derived by monitoring the resupply currentsupplied to the kinescope via the high voltage supply circuit of thereceiver as is known.

What is claimed is:
 1. In a video signal processing system including avideo signal processing channel, a kinescope for displaying imagesderived from processed video signals, and means for coupling processedvideo signals from said video channel to intensity control electrodes ofsaid kinescope, apparatus comprising:a source of signals containinggraphics information to be displayed by said kinescope; means coupled tosaid source of signals for deriving therefrom signals representative ofgraphics information to be displayed; bias means for providing agraphics intensity bias signal representative of a desired intensitylevel of displayed graphics information; first switching means having asignal input terminal coupled to said video channel, a signal outputterminal coupled to said coupling means, and a switching control inputterminal; second switching means having a signal input terminal coupledto said graphics bias signal, a signal output terminal coupled to saidcoupling means, and a switching control input terminal; and controlmeans responsive to output signals from said deriving means forproviding switching control signals to said switching control inputterminals of said first and second switching means, for (a) enablingsaid first switching means to conduct video signals to said couplingmeans, and disabling said second switching means to prevent conductionof said graphics bias signal to said coupling means in a normaloperating mode of said system when video signals alone are to bedisplayed, and (b) disabling said first switching means to preventconduction of video signals to said coupling means, and enabling saidsecond switching means to conduct said graphics bias signal to saidcoupling means during graphics display intervals in a graphics displayoperating mode of said system.
 2. Apparatus according to claim 1,wherein:said source of signals provides composite video signalsincluding encoded graphics information; and said signal deriving meansincludes means for decoding said graphics information contained in saidencoded video signal.
 3. Apparatus according to claim 1, wherein:saidfirst and second switching means comprise signal transmission gatesoperable between conductive and nonconductive states.
 4. Apparatusaccording to claim 1, wherein:said coupling means comprises a videooutput amplifier stage for supplying amplified signals to said intensitycontrol electrodes of said kinescope.
 5. Apparatus according to claim 4,wherein:said signal output terminals of said first and second switchingmeans are coupled in common to a signal input of said amplifier stage.6. Apparatus according to claim 2, wherein:said decoding means providesan output blanking signal including a first blanking level in a firstgraphics display mode when graphics information is to be displayedexclusive of video information, and a second blanking level in a secondgraphics display mode when graphics information is to be displayedtogether with video image information; and wherein said blanking signalis coupled to said video signal processing channel for inhibiting videosignals processed by said channel in said first graphics display mode,and for enabling processing of video signals by said channel duringimage line display intervals in said second graphics display mode. 7.Apparatus according to claim 6, and further comprising:third switchingmeans coupled between said signal output terminal of said secondswitching means and a point of reference potential corresponding to adesired black level of a displayed image, and having a switching controlinput responsive to said blanking signal in said first graphics displaymode, for rapidly connecting said output terminal of said secondswitching means to said reference potential at the end of graphicsdisplay intervals in said first graphics display mode.
 8. Apparatusaccording to claim 1, wherein:said bias means is coupled to said videochannel for monitoring the magnitude of video signals processed by saidchannel; and wherein said bias means further includes means for varyingthe magnitude of said graphics intensity bias signal in accordance withthe magnitude of said video signals.
 9. Apparatus according to claim 8,wherein said bias means further includes:means for establishing aminimum level for said graphics intensity bias signal; and means forestablishing a maximum level of said graphics intensity bias signal. 10.Apparatus according to claim 9, and further comprising:means coupled tosaid bias means for controlling the magnitude of said graphics intensitybias signal in a direction to reduce the magnitude of currents conductedby said kinescope in response to the magnitude of said graphicsintensity bias signal, when currents associated with said bias signalexceed a level otherwise sufficient to cause excessive currents to beconducted by said kinescope.
 11. In a color television receiverincluding a plurality of video signal paths for conducting a pluralityof color image representative video signals derived from a compositetelevision signal, said composite television signal subject to inclusionof encoded graphics information to be displayed; a kinescope fordisplaying images derived from said video signals and having a pluralityof intensity control electrodes; and a video output amplifier stagehaving a plurality of signal inputs, for coupling said plural videosignals to respective intensity control electrodes of said kinescope;apparatus comprising:means, including graphics signal decoding means,for decoding said television signal to derive therefrom decoded outputsignals representative of said graphics information to be displayed;bias means for providing a graphics intensity bias signal representativeof a desired intensity level of displayed graphics information; aplurality of video signal switching means, each having a signal inputterminal coupled to a separate one of said video signal paths, a signaloutput terminal coupled to a separate one of said video amplifierinputs, and a switching control input terminal; a plurality of graphicssignal switching means, each having a signal input terminal coupled tosaid graphics bias signal, a signal output terminal coupled to aseparate one of said video amplifier stage inputs, and a switchingcontrol input terminal; and control means responsive to output signalsfrom said deriving means for providing switching control signals to saidswitching control input terminals of said video and graphics signalswitching means, for (a) enabling said plurality of video switchingmeans to conduct video signals to said video amplifier, and disablingsaid plurality of graphics switching means to prevent conduction of saidgraphics bias signal to said video amplifier in a normal operating modeof said receiver, and (b) disabling said plurality of video switchingmeans to prevent conduction of video signals to said video amplifier,and enabling said plurality of graphics switching means to conduct saidgraphics bias voltage to said video amplifier during graphics displayintervals in a graphics display operating mode of said receiver. 12.Apparatus according to claim 11, wherein:said bias means is coupled tosaid plurality of video signal paths for monitoring the magnitude ofvideo signals conducted by each of said signal paths; and wherein saidbias means further includes means for varying the magnitude of saidgraphics intensity bias signal in accordance with the magnitude of videosignals conducted by said plurality of signal paths.
 13. Apparatusaccording to claim 12, wherein said bias means further includes:meansfor establishing a minimum level for said graphics intensity biassignal; and means for establishing a maximum level of said graphicsintensity bias signal.
 14. Apparatus according to claim 13, and furthercomprising:means coupled to said bias means for controlling themagnitude of said graphics intensity bias signal in a direction toreduce the magnitude of currents conducted by said kinescope in responseto the magnitude of said graphics intensity bias signal, when currentsassociated with said bias signal exceed a level otherwise sufficient tocause excessive currents to be conducted by said kinescope.